THE CURRENTS OF THE STRAITS.
The Mediterranean is like a bag with two necks filling at both ends.
The current through the Dardanelles presents exciting varieties, but no
perplexing mysteries. It is the discharge of the surplus of the Black
Sea, and the current is subject to the influences of the northerly
and southerly winds; being reversed when the latter long prevails. At
Gibraltar all is disorder--the stream incessant--the level on both
sides the same. The tide rises and falls, yet the current always runs
out of the ocean and into the Mediterranean. So determined is this
rush, that the gales of the Equinox neither quicken nor retard it, and
the phases of the moon have no power over it. It bursts through all
obstacles and transgresses all laws, and seems to move by a will of
its own--too strong to be disturbed, too deep to be discovered. During
my excursions I was engaged in examining these phenomena, and I will
commence with stating the results of several months’ cogitation and
inquiries.
I first applied myself to test the old explanation of an under-current,
by endeavouring to float substances at various levels, and after great
trouble in procuring lines, and having machines of various kinds made,
I found that without a frigate’s tackle and crew no results could be
obtained. I was thus reduced to mere scrutiny of the alleged facts, and
of the alleged theory. The facts amount to this: a vessel, in 1754,[24]
was fired into from the battery, it sank in face of the rock, and was
afterwards cast up in the bay of Tangier.
A vessel, when it sinks, goes to the bottom, and if fragments of it are
detached and are cast ashore, it is only because they float, that is,
they rise to the surface. This story will not, therefore, serve the
theory, even if authentic. There is nothing to prevent a ship or timber
from floating out; for close in shore, on both sides, the tides of the
ocean rise within the Straits to the height of four feet: of these,
boats take advantage to get through against both wind and current.
Sometimes, indeed, though it very rarely happens, the whole current
is reversed; and vessels working during the night, and reckoning on
being carried fifty miles to the eastward, have found themselves in the
morning ninety miles to the westward of the point where they expected
to be, that is to say, carried forty miles over the ground to the
westward during the night.[25]
Having thus disposed of the only, but incessantly quoted fact, I
proceed to the theory. Reasoning, however, there is none, for it
amounts to nothing more than this: “What becomes of all this water?
It cannot go to the Black Sea, from which the Mediterranean receives
water; it cannot escape by a subterranean passage into the Red Sea, for
the level of the Red Sea is higher by thirty feet. Then there is an
under-current discharging the water back again into the ocean.”
Water moves by its weight. Unless there is difference of level, there
is no motion. The resistance is from the bottom according to its
roughness, and the _vis inertiæ_ is felt at the top--thus the greatest
speed is at about two-thirds of the depth; here there is no difference
of level, nor is the water acted on superficially by any propelling
power. There is no prevalence of winds to account for a current at
the surface. So great is the momentum of the stream, that, unlike the
currents of the Dardanelles, it is neither accelerated by favourable
winds, nor even retarded by adverse storms. The idea of an over-current
running against an under-current is so opposed to all experience, that
to be admissible, proofs would be required, and it could never be
received as an hypothesis to account for an unexplained phenomenon.
Thus, the theoretical explanations utterly fail; yet there is action
without agent, momentum without motor, currents without winds or
declivity, and a vessel constantly filling without escape or overflow.
A mighty river rushes over its bed; but this river is not moved by
its weight; it runs on a dead level[26] to the sea it reaches from the
fountain whence it springs.--That fountain is the ocean itself! No
wonder that this should be the first of ancient mysteries, and the last
to be explained.
Before I had discarded the idea of an under-current, or had discovered
the insufficiency of the evaporation to account for the indraught, I
was sitting on Partridge Island, (a small rock within the Straits,) and
gazing with astonishment at the enormous mass of water running by me,
when the question occurred to me, _what becomes of the salt_? If the
water evaporate, the salt remains; here then is the sluice of a mighty
salt-pan--where is the produce? This has been going on for thousands
of years; is there a deposit of salt at the bottom? If so, why have
the abysses of the Mediterranean not been filled up? But salt is not
deposited; how then is the Mediterranean not become brine? Then I saw
that the evaporation would not account for the indraught, and before
I descended from that rock, I had solved the problem. That solution
is--an under-current produced by a difference of specific gravity
between the water of the Mediterranean and the ocean.
If you take two vessels, and fill one with fresh water, and the other
with salt, or the one with sea-water at its ordinary charge of 1030,
and the other with sea-water of higher specific gravity, such as
would result from evaporating a portion of it, say 1100, and colour
differently the water in the two vessels, and then raise a sluice
between them, you will instantly have two currents established in
_opposite directions_. In fact, you produce currents of water, like
currents of wind, by the converse of rarefaction.
“Recent discoveries,” says Humboldt, “have shown that the ocean has
its currents exactly as the air. Living, as we do, upon the surface,
they have been beyond our reach; but now, having obtained soundings to
the depth of four miles, we have ascertained that there is a rush of
icy water from the Pole to the Equator, just as there is a draft of
air close to the earth into the centre of Africa. The Mediterranean
offers an apparent anomaly of a higher temperature at great depths.
This Arago explains by the fact, ‘That the surface of the water flows
in as a Westerly current, whilst a counter current prevails beneath,
and prevents the influx from the ocean of the cold current from the
Pole.’[27] If there was nothing to determine the currents at the
entrance of the Mediterranean, save the relative degrees of cold at
great depths between it and the ocean, the cold water would run in at
the lowest depths, and the warm water would run out on the surface,
which is precisely the reverse of what it does.
Here is a body of water 740,000 square miles in extent, subject
hourly to the increase of its specific gravity. Upon the surface, a
crust of salt is left in the course of every year, sufficient to give
a double charge to the depth of six fathoms. To adjust the difference
thus created with the ocean, there is but a narrow inlet,--a mere
crack upon the side of the vessel, an interval of six miles left in
a circumference of four thousand. By this, in its deepest part, the
heavier water will have to find its way out, and thus occasion an
indraught of water above, besides the demand created by evaporation. It
remains to be ascertained by experiment, that the specific gravity of
the water in the Straits varies at different levels, and at what level
it commences to move outwards. These experiments will present great
practical difficulties from the tides at the sides, which will mingle
the streams; and, from the shallowness towards the ocean, they must be
made in the middle and at the Mediterranean side. The evaporation, and
the differences of specific gravity, will give the means of calculating
the amount of water passing through in both directions, and the depth
and velocity of the two currents. But it may be inferred that the
currents will have the greatest speed at the top and the bottom,--that
their velocity will diminish towards the centre, and that a neutral
space of dead water will remain, not only in consequence of the
counter-impetus of the currents, but because of the nearer approach
of specific gravity, and the mingling of the two waters, which would
destroy the moving power.
With this solution we can at once understand the powerlessness of
tides and storms, currents without difference of level, or prevalence
of winds: the volume of the stream is accounted for, the mass of salt
disposed of, and the apparent rebellion against the laws of Nature put
down.
By tables kept for several years at Malta, it appears that the
Mediterranean, at that point, varies in level between winter and summer
no less than three feet. In winter, when there is no evaporation, and
when the quantity of water falling in the immediate vicinity of the
Mediterranean is greatest, the level is lowest. The cause, I should
take to be the pressure of wintry wind. In like manner, those erratic
movements in the Straits may result from difference of atmospheric
pressure without and within.
These currents, by the testimony of the ancients, have not held from
the beginning--they have been the results of successive modifications
of the channel. This is singularly borne out by the traditions of the
neighbouring people and the geological features of the coast.
Eldressi narrates, as an old and popular story of his day, that “the
Sea of Cham (Mediterranean) was in ancient times a lake surrounded on
all sides, like the Sea of Tabaristan (Caspian), the waters of which
have no communication with any other seas. So that the inhabitants
of the extreme west invaded the people of Andalusia, doing them much
injury, which they, in like manner, did to the others, living always
in war, until the time of Alexander,[28] who consulted his wise men
and artificers about cutting that arid isthmus and opening a canal.
Thereupon they measured the earth, and the depth of the two seas,
and saw that the Sea of Cham was not much lower than the Sea Muhit,
(Ocean); so they raised the towns that were on the coast of the Sea
of Cham, changing them to the high ground. Then he ordered the earth
to be dug out; and they dug it away to the bottom of the mountains on
both sides; and he built there two terraces with stones and lime the
whole length between the two seas, which was twelve miles; one on the
side of Tankhe (Tangier), and one on the side of Andeluz. When this was
done, he caused the mound to be broken, and the water rushed in from
the great sea with violence, raising the waters of the Sea of Cham, so
that many cities perished, and their inhabitants were drowned, and the
waters rose above the dykes, and carried them away, and did not rest
until they had reached the mountains on both sides.”
The Moors have also a Myth. “The sea,” they say, “was created fresh,
but exalting itself against its Maker, gnats were sent to drink it up.
It then humbled itself in the stomach of the gnats, and prayed to be
relieved, so the gnats were ordered to vomit it forth again, but the
salt remained from the stomach of the gnats an eternal sign of its
disobedience.”
Before suggesting the interpretation of these Myths, I will point out
the change which the coasts and channel have undergone.
The description of the Straits by Greek and Roman writers is so unlike
their present appearance, that, but for the impossibility of doubting
the identity of the objects, we must have supposed their words to apply
to some undiscovered region. Who could recognize the deep sea and the
iron-bound coasts of these narrows, in a plain of sand furrowed by
rivers running in from the ocean, which it was difficult to reach,
not from the strength of the stream, but the intricacies of the
passage--who would imagine the necessity of constructing flat-bottomed
boats to get across from Gibraltar to Ceuta, where now there is above
one thousand fathoms, or of transferring the ferry to the Atlantic
side of the Straits, where at present the depth is not one-sixth of
what it is in the other, in order to get more water? Yet there is no
doubt that these details apply to these spots, nor can we question the
known accuracy of the writers, or escape from the concurrence of their
testimony. We are reduced, then, to the necessity of admitting some
great revolution in the features of the country, and a total change in
the nature of the current.
The explanation is easy: the bank of sand, left by the retiring waters
of the Deluge, which covers the western border of Africa, reached to
the coast of Andalusia, and the remnants of it still lie on the eastern
side of Gibraltar, and fill the caverns exposed to that side; on the
depression of the Mediterranean by evaporation, the water of the ocean
would filter in, the sand would be gradually removed. The amount of
sand on the Mediterranean side of the “Rock,” shows that a plain nearly
one thousand feet above the level of the sea, once stretched across to
Africa, where now the channel is one thousand fathoms deep. This has
been worked out by the overfall, while on the Atlantic side the water
shoals to one hundred and eighty fathoms, presenting the character of
an estuary with a bar from the rush outward of the under waters, since
the Gut was sufficiently deepened to admit of the currents in opposite
directions. The centre part of the channel is worn down to the rock or
to gravel, every particle of sand has been removed from the bottom. Two
inferences may be drawn: 1st. That the process of removal was likely to
be accompanied by sudden inbursts, which would submerge the borders.
2nd. That the Mediterranean, in early ages, was fresh, and afterwards
became, as it evaporated, very salt, until the channel was deepened to
allow of its mixing with the ocean. What else is implied by the Myth of
the midges and the fable of Alexander?
FOOTNOTES:
[24] See James’s History of Gibraltar.
[25] This happened to the _Phantome_.
[26] The excellent geodesic operations of Corabœuf and Deleros
have shown, that at the two extremities of the Pyrenæan chain, as
well as at Marseilles and the northern coast of Holland, there is
no sensible difference between the level of the Atlantic and the
Mediterranean.--_Cosmos_, vol. i. p. 297.
[27] _Cosmos_, vol. i. p. 296.
[28] In eastern tradition there are two Alexanders: the first is
Dualkernein, whom the Bretons claim as their leader from the Holy Land,
and the opponent of Joshua. According to the authorities cited in
Price’s Arabia (p. 54), the first Alexander was also a Macedonian, and
built a city in Egypt, on the site of the city afterwards raised by the
Macedonian. The ramparts of brass at the Caspian gate were attributed
to both Alexanders--they are by the Koran given to the first. (Sale’s
Koran, ch. xvii. p. 120; Merkhond’s Early Kings of Persia, p. 368). Al
Makkari says, the same Alexander built towns of brass in the Canary
Islands. Macarius, patriarch of Antioch, speaks of the Dardanelles
being opened by Alexander, and that he placed his own statue on the
top of one of the hills (Travels, vol. i. p. 33-40). Thus confounding
together the deluge of Ogyges--the cutting the canal of Athos--the
opening the Straits of Gibraltar. Alexander seems to have adopted the
title of the two former to favour the analogy (see Merkhnd, 334; Price,
49; Temple of Jerusalem, p. 119). Alexander Dualkernein, is still a
hero of the Spanish nurseries.